1. Field
The present disclosure relates generally to testing of material to determine thermal conductivity of a material or system of materials.
2. Background
In today's world of increasing demands for energy and energy efficiency, the use of cryogenics and refrigeration is taking on a more and more significant role. From the food industry, transportation, energy, and medical applications to the Space Shuttle, cryogenic liquids and other refrigerants must be stored, handled, and transferred from one point to another without losing their unique properties. To protect storage tanks, transfer lines, and other process system equipment from heat energy, high-performance materials are needed to provide effective thermal insulation to a degree that can be reasonably obtained. Complete and accurate thermal characterization of the insulation material, i.e., performance attributes of the material such as thermal conductivity and heat flux, is a key aspect in designing efficient and effective low-maintenance cryogenic and low-temperature systems.
One valuable technique for testing the thermal performance of materials, such as insulation material, is evaporation or boil-off testing. Boil-off testing is accomplished by filling a vessel with a fluid which evaporates or boils below ambient temperature. In the general sense, boiling is associated with higher heat transfer rates and evaporation with lower heat transfer rates. Although the exemplary fluid is the cryogen liquid nitrogen, other fluids such as liquid helium, liquid methane, liquid hydrogen, or known refrigerants may be used. A vessel is surrounded with the testing material, placed in a suitable environmental chamber, and then filled with the test fluid such as a cryogenic liquid. A calorimetry method is then used to determine the thermal conductivity of the test material by first determining the rate of heat passing through the test material to the vessel containing the refrigerant liquid. The heat leakage rate passing through the test material to the liquid in the vessel is directly proportional to the liquid boil-off rate from the vessel. For a test material under a set vacuum pressure, the effective thermal conductivity (k-value) and/or heat flux is determined by measuring the flow rate of boil-off at prescribed warm and cold boundary temperatures across the thickness of the sample.
Although other cryogenic boil-off techniques and devices have been prepared to determine the thermal conductivity of insulation material, the previous techniques and devices are undesirable for a variety of reasons. First, few such cryogenic devices are in operation because of their impracticality from an engineering point of view. The previous boil-off devices made it extremely difficult to obtain accurate, stable measurements and required extremely long set up times. Prior testing devices also needed highly skilled personnel that could oversee the operation of the testing device for extended periods of time, over 24 hours to many days in some cases. Additionally, constant attention was required to operate previous testing devices to make the necessary fine adjustments required of the testing apparatus. Second, prior testing devices contained the limitation that they did not permit the testing of continuously rolled products which are commonly used insulation materials. The testing of high-performance materials such as multilayer insulation requires extreme care in fabrication and installation. Inconsistency in wrapping techniques is a dominant source of error and poses a basic problem in the comparison of such materials. Improper treatment of the ends or seams can render a measurement several times worse than predicted. Localized compression effects, sensor installation, and outgassing are further complications. Third, measurements of various testing parameters were not carefully determined or controlled in previous testing devices. Measurement of temperature profiles for insulation material was either not done or was minimal because of the practical difficulties associated with the placement, feed-through, and calibration of the temperature sensors. Vacuum levels were restricted to one or two set points or not actively controlled altogether. Fourth, previous cryogenic testing devices required complex thermal guards having cryogenic fluid-filled chambers to reduce unwanted heat leaks (end effects) to a tolerable level. The previous technique for providing thermal guards, filling guard chambers with the cryogen, caused much complexity both in construction and operation of the apparatus. Known techniques add the further complication of heat transfer between the test chamber and the guard chambers due to the thermal stratification and destratification processes of the liquid within the chambers.